Generally, a fingerprint verification device is used to compare a fingerprint image, which has been collected from a person to be identified, with a registered fingerprint image in order to determine whether the person to be identified is in the register. In such types of fingerprint verification devices, the fingerprint image collected from the person to be identified may be of poor quality, such as an image containing inadequate fingerprint ridgeline information caused by a dry or wet finger. In such case, sufficient information about the individual cannot be extracted in order to differentiate the person to be identified from the registered fingerprint images, thereby resulting in erroneous verification. Consequently, it has become an important issue to improve verification precision by using only good quality fingerprint images (i.e., fingerprint images having sufficient fingerprint ridgeline information and fingerprint images having little noise). In order to solve this problem, it is essential to evaluate the quality of the fingerprint image; thus, several technologies for evaluating the quality of fingerprint images have been previously developed.
In one of these technologies, the dark and light portions of the fingerprint image are represented by binary numbers, and the area of the dark portion (or light portion) of a predetermined area within the binary image is obtained. The quality of the fingerprint image is evaluated based upon the ratio of the dark area to the light area (e.g., Japanese Laid-open Patent Publication No. 8-110949). In another method, the number of characteristic points (endpoints or bifurcation points of the fingerprint ridgelines) of the fingerprint image is obtained. The quality of the fingerprint image is evaluated based upon the number of obtained characteristic points (e.g., Japanese Laid-open Patent Publication No. 8-129644).
However, the method for evaluating the quality of the fingerprint image using the ratio of dark and light portions makes the determination of the quality of the fingerprint image based solely upon the ratio of dark and light portions of the fingerprint image. Fingerprint ridgelines are not taken into consideration. Consequently, images containing insufficient data representing the fingerprint ridgelines (i.e., images having warped or collapsed fingerprint ridgelines) are sometimes determined to be good quality images. On the other hand, the technology for determining the quality of the fingerprint image by using characteristic points requires a long time to extract the characteristic points from the fingerprint image. Therefore, it is not easy to determine the quality of the fingerprint images.
Accordingly, it is an object of the present invention to provide fingerprint image evaluation methods and fingerprint verification devices capable of evaluating the quality of fingerprint images using a simple process that utilizes the characteristics of fingerprint ridgelines.